Are the things we know true

Brain research : The great illusion

Close one eye. Do you now see a hole in the text, a black spot on the newspaper page? No? Then you have every reason to be suspicious. Because there is such a spot on her retina. Because the retina is built the wrong way round, with the sensory cells facing inwards and the nerve processes facing outwards towards the light, it needs this hole through which the collected nerve tracts, like a thick cable, send the visual information towards the brain. Nevertheless, no one perceives such a black spot in the field of vision because the brain fills the gap and leads us to believe that the world is perfect.

So distrust of our brain is appropriate. Even psychologists have had to learn that with great effort. For a long time they were particularly enthusiastic about the capabilities of the human mind, how well we see colors, how precisely we grasp objects, how well we recognize patterns. But it is now clear: not everything we see, feel and hear is also reality. “If you see a woman in the distance who reminds you of your wife, then you will think you recognize her much faster than you actually see her. The brain supplements the missing information, ”says Peter Falkai, psychiatrist at the Göttingen University Clinic.

The brain tries to provide a coherent model of the world as possible. What does not fit is made to fit. To do this, the brain not only fills in gaps, it also calculates what to expect in the future. It's easiest with what we do ourselves. “If someone turns off the light for a moment, then of course we notice. But when we blink, we don't notice that the perception is briefly switched off, ”says John Dylan-Haynes from the Bernstein Center for Computational Neurosciences in Berlin. Because the brain itself gives the command to blink, it can also predict brief loss of field of vision - and ignore it. The consciousness is simply bypassed.

The same applies to every eye movement. Close one eye again, then move your other eyeball up and down with one finger. It looks like the world is moving up and down. Now look up and down again. Although the same thing happens on the retina, in the second case it doesn't feel like the world is moving. There is a simple explanation for this: the brain not only sends the command to move to the eyes, it also sends a copy to the centers of perception. “So the brain knows that it has to expect movement and suppresses the perception of this movement,” says Haynes. It does this with every movement, with every step, with every movement. Therefore, it is also not possible to tickle yourself. The brain is always one step ahead of itself.

Patients with psychiatric illnesses show how important this is. For example, some people who suffer from schizophrenia experience a particularly disturbing symptom, the delusional influence. These people feel that their movements are controlled by foreign forces, but that they themselves have no control over their limbs. The theory: Because the announcement of the movement in the brain does not reach its destination, these people perceive their own movements in the same way as healthy people only experience movements that someone else performs with them. Much speaks in favor of this thesis. Chris Frith, a neuropsychologist from University College London, was able to prove that schizophrenic people can also tickle themselves.

But the brain makes constant predictions not only about its own body, but also about the outside world - and is then primarily interested in things that turn out differently than expected. Scientists at the Max Planck Institute (MPI) for Brain Research in Frankfurt published a study in February in which they showed that our brain is primarily active when it cannot predict a visual stimulus. The researchers showed test subjects a moving bar on a screen. At the same time, they measured activity in the part of the brain that processes visual information. There they always found a strong signal when a bar broke out of the expected movement pattern. “We conclude that the brain doesn't just wait for signals from the sense organs. Instead, it actively tries to predict possible sensory impressions. If the predictions are correct, the brain can process the information that actually arrives particularly effectively, ”says MPI Director Wolf Singer.

This applies not only to what we see, but also to what we hear. If you play the same note to a person at regular intervals, the brain shows little activity. But as soon as a tone is slightly higher or lower than the previous one, the brain becomes active. Researchers have been able to show something very similar in the case of spoken language. People show their greatest brain activity when a word in a sentence is surprising. "With a sentence like 'The toaster is happy' you suddenly see a strong signal with the last word," explains Frith.

And a few days ago the brain researcher Antonio Damasio published a study in the journal "Nature Neuroscience" in which he showed that the part of our brain that processes noise is active even when there is nothing to hear. Damasio showed eight test subjects short silent films that showed, for example, a crowing rooster, a howling dog or a breaking vase. At the same time, he measured the activity in the auditory center of the brain. It was active on all test subjects, although it was completely silent when the films were shown. Internally, the test subjects obviously heard the rooster and the dog. In fact, the patterns were so different that the researchers were able to predict whether the test person had seen an animal, an instrument or another scene based on the brain signals alone.

The usual image of perception - stimuli from the outside world hit the sensory organs, these pass information on to the brain, which uses it to create a worldview - is therefore out of date. In his impressively clear book “How our brain creates the world” (Spektrum-Verlag, 312 pages, 24.95 euros), Frith sums it up in a simple formula: What we perceive is a fantasy image that coincides with reality. “My perception is a prediction of what should be in the outside world,” says Frith. This prediction is then compared to what the senses actually report to the brain. In the ideal case, errors in the prediction are found and thus lead to the predicted worldview being adapted and improved.

But where do the predictions about the world come from? Some were created over millions of years and are part of the basic cognitive equipment with which we are born. For example, that light comes from above or that faces are turned outwards. That is why we also perceive the inside of a hollow mask as if it had been turned inside out. Our brain cannot see anything other than a normal face. Because it has learned over millions of years: there are no faces turned inside.

Other predictions are based on our own experience. For example, if a red wine glass is on the table in front of us, the brain calculates the taste to be expected. We are accordingly surprised when we put the glass to our lips and it suddenly tastes sweet.

“We now know how much the brain does without us ever being aware of it. That's a lot more than we would have believed earlier, ”says Frith. So a whole new question arose: "If the brain unconsciously does all these things so well, what do we need consciousness for?"

The psychologist also has a simple answer ready: We are aware of certain events and decisions so that we can talk about them. “We can't talk about things that happen unconsciously. But if our communication is to be of benefit, then we have to be able to share our experiences, ”says Frith.

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